1. Technical Field
This invention relates to motor vehicle speed control systems.
2. Prior Art
There are known many apparatuses for a motor vehicle speed control. For example, U.S. Pat. No. 3,804,193 issued to Ikuta teaches an automatic constant speed control system for vehicles. U.S. Pat. No. 4,138,723 issued to Nehmer et al teaches a motor vehicle speed control system which is microprocessor-based.
Some such speed control systems use electrical solenoids to activate the application of a vacuum to or the venting of the vacuum from a pressure reservoir, or actuator chamber, in order to move a diaphragm mechanically coupled to the throttle. Typically, such a solenoid is coupled in series with a switch to control activation of the solenoid. However, the operation of the switch can be thwarted by shorting various portions of the circuit directly to ground potential or to a battery potential.
In some known speed control systems, battery voltage is applied to the solenoids controlling the vacuum valves and the vent valves at all times. This means that a short to ground in the electrical lines going to the vacuum and vent solenoids would activate the actuator chamber even if the speed control module were turned off. In fact, a short in both the lines to the vacuum and vent solenoids at the same time would cause the speed control actuator chamber to go to a state where the vacuum solenoid is energized causing the vacuum valve to be open and the vent solenoid is energized causing the vent valve to be closed. As a result, the actuator chamber pressure is at a vacuum pressure and the throttle is pulled to an open throttle position, an undesired condition when the speed control module is turned off. These are some of the problems this invention overcomes.